In the second installment of Remembrance of Earth's Past (AKA the 3-Body Problem series), The Dark Forest, one of the scientific advances is a fission-driven spacecraft. That is that a "non-chemical propulsion" spacecraft is created, in fact capable of reaching 15% of the speed of light, and that observing the thrusters is like seeing a small star.

Is there any indication in the novel how this drive is supposed to work? Is the premise, for example, that radioactive byproducts are focused and ejected in a beam at a near-speed of light velocity (accelerating the spacecraft by conservation of inertia) or something else entirely?

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    I don't know the story, but maybe it's a variation on en.wikipedia.org/wiki/Nuclear_salt-water_rocket
    – PM 2Ring
    Commented May 20, 2022 at 11:05
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    Are you confusing the fission-drive program with the fusion drive? I think it was the fusion drive that was described as looking like "a small sun'. Commented May 20, 2022 at 12:59
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    Clara may well be right. The text says "Natural Selection was the flagship of the third squadron of the Asian Fleet, and in gross tonnage and performance it was second to none. Possessing the latest non-media fusion propulsion system, at full thrust it could accelerate to 15 percent of the speed of light...". Commented May 20, 2022 at 17:05
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    Also, "Three years after the breakthrough in controlled nuclear fusion, new and unusual heavenly bodies had taken their place in the Earth’s night sky, up to five of them now simultaneously visible in one hemisphere. The bodies changed dramatically in luminance, outshining Venus at their brightest, and often blinked rapidly. Sometimes one of them would suddenly erupt with a rapid increase in brightness, then go out after two or three seconds. They were fusion reactors undergoing tests in geosynchronous orbit.". Commented May 20, 2022 at 17:11
  • @DoscoJones thanks this is the part I was interested in. Mainly I was interested why it should look like a star, presumably emitting light from all sides, like uncontrolled fission occurring in open space, instead of nothing or perhaps a beam of light, since the reactor would be contained and shielded extensively, except for the nozzle from the jet exit's sit.
    – AdamO
    Commented May 20, 2022 at 17:38

1 Answer 1


This is a very old idea. From the Wikipedia article on the NERVA project:

Nuclear rocket engines use a nuclear reactor to provide the energy to heat the fuel instead of a chemical reaction. Because nuclear reactions are much more powerful than chemical ones, a large volume of chemicals can be replaced by a small reactor. As the heat source is independent of the working mass, the working fluid can be selected for maximum performance for a given task, not its underlying reaction energy. For a variety of reasons, hydrogen is normally used. This combination of features allows a nuclear engine to outperform a chemical one; they generally aim to have at least twice the specific impulse of a chemical engine.

  • Is there any indication in the novel that this is what is meant by a fission engine (as opposed to, for example, some manner of scifi drive like 'impulse engines').
    – Valorum
    Commented May 20, 2022 at 16:44
  • An admittedly quick check of the text only indicates that fission drive tech was being worked on before fusion tech became workable. In any case, I think the original question is flawed; see the comments from Clara and myself. Commented May 20, 2022 at 17:19
  • The fission drives were "media propelled", and so working by heating up fuel and throwing out the back, just like NERVA. The drive described in the question seems to be from a fusion drive though, described in the text as "non-media propulsion", and so would work very differently. As @DoscoJones says, the question is not clear in its current form - there is a big difference between fission and fusion! Commented May 20, 2022 at 19:43

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